Radio repeaters are commonly used to enhance the radio connectivity in harsh communication environments where the effects of the communication channel such as multipath, attenuation and non-line of sign have an adverse impact on the connectivity of the channel. A radio repeater can assist reestablishing the line-of-sight between the transmitter and receiver in such environments. In addition, a radio repeater can be used to extend the communication range by amplifying and retransmitting the signal along a desired direction. Most radio repeaters down-convert the signal and then up-convert and retransmit at a different frequency. The advantage of using this theme is that the transmit (Tx) and receive (Rx) channels are isolated through the system diplexer. However, a significant portion of the power used by these systems is devoted to changing the carrier frequency and only a small fraction of the power used by the system is the radiated power. In addition, such repeaters must rely on a communication protocol to be able to receive and transmit in separate channels which increases the complexity of the repeater significantly as well as decreasing the repeater's compatibility with different devices and the number channels that can be simultaneously supported. Alternatively, a protocol independent low power repeater that consists only of Tx and Rx antennas and a low power high gain amplifier can be used. Using this theme, all communications from multiple users and different modulation schemes in the frequency band of the repeater can be supported simultaneously. This way no protocols for routing the data packets of different users need to be satisfied and the full bandwidth can be utilized with high power efficiency. The main challenge in implementing this configuration pertains to the coupling level between the adjacent Tx and Rx antennas operating at the same frequency band. The mutual coupling between the two antennas creates a feedback which if higher than the reciprocal of the amplifier gain, can cause the system to go to oscillation. This coupling imposes a restriction on how high the amplifier gain can be chosen. Several approaches have been proposed to decrease the mutual coupling between the antennas. These approaches include modifying the currents on the ground plane on which the antennas are fabricated, using engineered electromagnetic structures such as electromagnetic band baps (EBG), or creating destructive interference between the two antennas. The defected ground structures improve the isolation but degrade radiation polarization and pattern. EBG structures usually require periodic geometry and large dimensions. Destructive interference between the antennas requires an odd multiple of half wavelength distance between the two Tx antennas as well as accurate, non-symmetric power distribution between the two Tx antennas.
Recently a protocol independent omnidirectional radio repeater for ad-hoc communications at IMS based was introduced. While omnidirectional repeaters provide good coverage in all directions and can handle forward and backward links simultaneously, they are not beneficial if the path loss in the channel is not significant as will be shown later in this disclosure. Moreover, as will be shown, the signal level must decrease by 10 dB every time the signal passes through a repeater if a 20 dB signal to noise ratio throughout the communication path is desired. In this disclosure, a directional repeater system is presented with much improved Tx and Rx antenna isolation. The isolation improvement is achieved by separating the antennas by a ground plane in between and also creating a polarization mismatch between the two antennas. This technique exploits the fact that most mobile platforms that use the IMS band to communicate are designed to be able to transmit and receive in both vertical and horizontal polarizations.
This section provides background information related to the present disclosure which is not necessarily prior art.